Yesterday we lost our one year old Australorp, Peppa. The day before she came out of the hen house with the rest of our flock and seemed normal at breakfast. A little while later we went back down to the coop and noticed that she was letting out a runny, stinky green-brown poop. Then she went to lay down in the corner of the coop and didn't move from that spot for the rest of the day. A little later, we noticed she was drooling from her beak and her comb was purplish. The other hens started picking on her so we took her into the garage in a temporary run we set up for her. She appeared very weak and we looked up on the internet (this site included) her symptoms. She laid an egg the day before so probably not egg bound. The drool didn't stink. Her crop felt normal. There was no wheezing, no beak discharge, no runny eyes. We gave her some water with apple cider vinegar and a little scrambled egg since she didn't eat anything since breakfast (and she was normally a good eater). We also gave her some softened food pellets too. Early in the day, she kept down what we fed her and she seemed to perk up. In the evening however, she vomited up the food and started looking week again. A woman who sells and keeps chickens nearby told us she might have eaten something toxic (maybe a fungus in the coop since it's been so wet in our area lately). She recommended a crushed up charcoal tablet, which we fed her. We stayed up with her as long as we could and she was still alive when we went to sleep. By the morning though she'd died. When we finally could get hold of the only vet around here who specializes in chickens, he seemed perplexed too and said it was either heart failure or pneumonia. We're wondering now if that could be correct and what measures we need to take to keep the rest of the flock healthy. Any advice would be much appreciated.